Each year an individual is exposed to approximately 6.2 mSv of ionizing radiation, half of which comes from medical diagnostic procedures. In addition to the standard hospital-based diagnostic radiation equipment such as x-ray machines, computed tomography (CT) scanners, positron emission tomography (PET) and nuclear medicine procedures, new ionizing radiation based diagnostic centers for heart scan, virtual colonoscopy and whole-body scan are opening at a rapid rate throughout the world, especially in the United States.
A computed tomography (CT) scan uses ionizing radiation to generate 3D views of internal organs and structures within the body. This form of medical imaging is primarily used to assist in the diagnosis of both acute and chronic medical conditions. The use of CT scans has increased rapidly over the last decade. In 2006 it was estimated that nearly 70 million CT scans were obtained in the US, as compared with approximately 3 million in 1980. The number of CT prescribed scans continues to rise, with 100 million annual scans projected in the US by 2015.
A typical CT scan generates a skin dose in the range of 10-50 mGy. Absorbed doses from multiple CT scans can reach levels that have been shown to induce a low rate of cancer incidence. In the case of CT angiography, typical organ doses from a single CT scan range from 10-25 mGy to as high as 100 mGy to the heart and lung. The rate per capita of CT scan usage in the United States has been estimated to be approximately 220 scans annually per 1000 population, which on a per capita basis is second only to Japan in the developed world. A rise in implementation along with cumulative effects from multiple CT scans brings risks from increased exposure to ionizing radiation.
Ionizing radiation causes DNA double strand breaks. Improper or inefficient repair of these harmful insults can result in an increased risk of birth defects in the event the individual is pregnant at the time of exposure, and development of cataracts and/or cancer later in life. Estimates have suggested that the risk of a fatal cancer occurrence from a single CT scan is about 1 in 500 for children and 1 in 2000 for adults. This is further compounded by increased usage of CT scan procedures, potentially increasing the burden on future health care systems.
Of significant concern is the rise in the use of CT scans in pediatric patients, who are at greater risk for developing cancer throughout their lifetime due to diagnostic or treatment-related exposure to radiation during childhood. One analysis found a 92% increase in CT scans of the abdomen and pelvis performed on children under the age of 15 between 1996 and 1999. Brenner, D., Elliston, C., Hall, E. and Berdon, W. (2001) Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT. AJR. American Journal of Roentgenology, 176, 289-296.
Children are not only more sensitive to the effects of radiation but also have a longer life expectancy, the combination of which further increases their risk of cancer in adult life. One retrospective study found that in children receiving CT scans, a cumulative dose of about 50 mGy has the potential to almost triple the risk of leukemia and doses of about 60 mGy may triple the risk of brain cancer. Pearce, M. S., Salotti, J. A., Little, M. P., McHugh, K., Lee, C., Kim, K. P., Howe, N. L., Ronckers, C. M., Rajaraman, P., Sir Craft, A. W. et al. (2012) Radiation Exposure from CT Scans in Childhood and Subsequent Risk of Leukaemia and Brain Tumours: a Retrospective Cohort Study. Lancet, 380, 499-505.
Not only are cells directly in the path of ionizing radiation at risk, surrounding cells are also affected by radiation exposure. The “radiation-induced bystander effect” has significant implications for low-dose radiation exposure. In one study, a majority of the bystander cells (40-60%) were found to have DNA double strand breaks. Sedelnikova, O. A., Nakamura, A., Kovalchuk, O., Koturbash, I., Mitchell, S. A., Marino, S. A., Brenner, D. J. and Bonner, W. M. (2007) DNA Double-Strand Breaks Form in Bystander Cells after Microbeam Irradiation of Three-Dimensional Human Tissue Models. Cancer Research, 67, 4295-4302. These direct and indirect biological effects of low-dose radiation are of significance when assessing total cancer risk. In addition, one group found the number of DNA double-strand breaks measured in blood lymphocytes following CT scan examination to be directly related to the dose-length product. Lobrich, M., Rief, N., Kuhne, M., Heckmann, M., Fleckenstein, J., Rube, C. and Uder, M. (2005) In Vivo Formation and Repair of DNA Double-Strand Breaks After Computed Tomography Examinations. Proceedings of the National Academy of Sciences of the United States of America, 102, 8984-8989. These data suggest that efficient means to resolve DNA double strand breaks and other biological damage in cells directly exposed to radiation and more importantly in bystander cells are of importance to mitigate biological-related deleterious effects.
Accordingly, a substantial need exists for a cost-effective method of reducing the detrimental effects of low dose radiation exposure.